Various types of display devices, including but not limited to liquid crystal display devices, electrochromic displays, plasma displays, field emission displays and electroluminescent displays include transparent conductive coatings formed thereon. These transparent conductive coatings have several purposes, including the creation of circuits within these display devices. Transparent conductive coatings are also used as antistatic coatings on instrument panels, heating elements on aircraft windows and electrodes on solar cells. These materials can also be used as heat retaining and ultraviolet light-rejecting coatings on windows.
Transparent conductive oxide coatings operate at lower voltages and remain transparent at a thickness in the range of microns instead of nanometers as compared to gold conductive coatings. As such, transparent conductive oxides extend the performance and thickness threshold before becoming too opaque to visible light.Generally, in the applications mentioned above, the transparent conductive film requires high visible-light transmittance in addition to high conductance, superior durability, and process ability such as coating or film formation. It is important to control film thickness and homogeneity. Generally, there are two ways to control film thickness and homogeneity. One way is to provide a process that creates thin and homogeneous coatings. These processes include, but are not limited to, chemical vapor deposition (CVD), sputtering and evaporation-condensation techniques. A second way to control film thickness and homogeneity is to provide materials that facilitate the formation of thin and homogeneous coatings. Sputtering processes require tight controls on raw materials. This disclosure focuses on the creation of materials that encourage thin and homogenous coatings.
Evaporation and condensation techniques are used to deposit gold and other transparent conductive oxide materials. These processes can be slow, time consuming and require tight controls of the environment. Sputtering these materials can be more forgiving since the material is not as intimately reacting with the environment.
Sputtering techniques also provide for the deposition of larger quantities of a material within a specific time period. Transparent oxide sputtering targets are made by traditional ceramic processing techniques and include ceramic crystals in the targets result in the sputtering of these crystals during the deposition process. If the crystal size is too large, the sputtered thickness will be too thick, making the film opaque. Chunks of material also prevent even current distribution in the sputtered film, negatively impacting the performance of the device.
Ceramic powders are usually an order of magnitude larger than the intended coating. Traditional ceramic processing of ceramic powders has been used to create transparent conductive oxide sputtering targets. The larger particle size of traditional ceramics extend the diffusion time required for the diffusion of dopants into the crystals. Since the film thickness is usually on the order of microns and ceramic powders are commonly ground to sizes on the order of 10's of microns, grinding transparent conductive materials requires challenging and tedious processing. The grinding, target-forming, and application processes must overcome the complications associated with the larger particle sizes of the raw material. Porosity in sputtering targets formed by traditional ceramic processing techniques also creates a problem.
Processing materials to fine particle sizes also creates several problems in traditional ceramic processing. Fine particle size powders have significant surface energy and agglomerate into porous clusters. These clusters take time and energy to sinter out. Sintering occurs at temperature near grain growth, which is the initial issue. If the porosity is not removed, these clusters can be sputtered instead of individual grains. Clusters create several surfaces that reflect and scatter light. Reflection and scattering decrease the transparency of the material. Thus, there continues to be a need to provide improved processes for forming transparent oxide material targets.